Solubilized PB-induced rat hepatic mitoplases (mitochondria without outer membrane hence with minimal microsomal contamination) containing mitochondrial cytochrome P-450 was chromatographed by successively on omega-aminoacetylsepharose 4B, DEAE sephocel, CM sepharose, and hydroxyapatite. Each chromatography was eluted with phosphate buffers of appropriate pH. Cytochrome P-450 content was determined by measuring the CO-difference spectra. The molecular weights of the cytochromes P-450 that were eluted from each column was monitored by SDS-PAGE. Four forms of mitochondrial cytochrome P-450 were isolated. The CO-difference maxima were 448-5, 449.0 449.5 and 450.0 nm.